Structural characterization of Ag-doped TiO2 with enhanced photocatalytic activity

Ag-doped TiO2 nanoparticles with different metallic content (0.5, 2.0, and 5.0% m/m) were prepared using a simple and cost-effective method based on the sol-gel technique, followed by thermal treatment. The addition of Ag+ ions during the hydrolysis/condensation of the Ti(IV) molecular precursor leads to homogeneous dispersion of the Ag+ cations on the titania matrix. As the amount of silver is increased, the resulting TiO2 nanoparticles exhibit smaller particle size (from 27 nm for bare TiO2 to 12 nm for TiO2-Ag 5.0%) and larger surface area. X-ray photoelectron spectroscopy (XPS) confirms that during the sintering step of the resultant powder at 400 degrees C for 5 hours, ca. 34% of the silver content is converted to Ag-0 via thermal decomposition of Ag2O. The data also indicates the presence of highly oxidized silver species, such as Ag2+ and Ag3+. X-ray diffractograms and Raman spectroscopy confirm that the crystalline structure of the TiO2 matrix corresponds to the anatase polymorph; however, the presence of the dopant leads to an increase in the system disorder due to a higher concentration of oxygen vacancies, as also confirmed by XPS. TiO2-Ag 5.0% exhibited the highest photocatalytic activity towards mineralization of the E102 tartrazine azo-dye, being 78% faster than bare TiO2 at optimum pH conditions (pH = 6.9). Upon light excitation, the oxidized silver cations are reduced to Ag-0, leading to an improvement in visible light absorption due to the surface plasmon resonance effect. The recycling of the photocatalyst showed that the enhanced photocatalytic activity is maintained, which can be associated with the reduction of charge recombination at the oxide surface and the enhanced visible light harvesting.